1. Field of the Invention:
The present invention relates to a transistor circuit used in a logic circuit, and more particularly to a transistor circuit in which collector saturation voltage is controlled or restricted.
2. Description of the Prior Art:
In a logic circuit, a signal has two levels of either "1" or "0". A transistor receiving this signal is driven into either a conductive state or a cut-off state. Accordingly, when the transistor is driven into the conductive state, a large volume of base current flows, which results in the so-called collector saturation state. If collector saturation has taken place, electrical charges are accumulated in a base region. Under such collector saturation state, if the input signal is changed to drive the transistor into the cut-off state, the transistor cannot change its electrical state to the cut-off state until this accumulated charges have been discharged. Consequently, there is a drawback in that the operable speed of the transistor circuit is limited. This drawback becomes more serious at the output stage where the transistor is strongly driven into the saturated state in order to obtain a large output current.
As a means of controlling such excess collector saturation, there has been known a method of connecting a Schottky diode between the collector and the base. In other words, this method ensures that the collector potential does not drop below the forward biased voltage of the Schottky diode from the base potential even if the base-emitter junction is strongly forward-biased so that a large base current will flow. Between a PN junction and a Schottky junction, the Schottky junction produces a lower forward biased voltage, so that the collector potential does not drop below the emitter voltage. For this reason, surplus input current is bypassed to the collector via the Schottky diode, with the result that an excess base current does not flow and the volume of the charges accumulated in the base region is restricted. Accordingly, it is possible to restrict the time for discharging the accumulated charges and to accelerate the operating speed.
However, when incorporating such a Schottky-clamped transistor circuit into an integrated circuit, it requires an additional step of forming the Schottky junction electrode, in addition to the usual steps of forming an integrated circuit. As a result, an increase in the number of manufacturing processes and changes of the manufacturing conditions are not available. This results in shortcomings of increasing production costs and a drop in the production yield. In addition, there is another drawback in that the collector saturation voltage cannot be arbitrarily controlled, since the forward voltage of the Schottky diode is self-determined by the metallic material of the Schottky electrode.